The subject matter disclosed herein relates generally to magnetic resonance imaging (MRI) systems, and more particularly to systems and methods for mapping radio-frequency (RF) fields for MRI systems having multiple transmit channels.
MRI or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between RF waves and nuclei in a magnetic field. Specifically, MRI utilizes hydrogen nuclear spins of the water molecules in the human body, which are polarized by a strong, uniform, static magnetic field of a magnet. This magnetic field is commonly referred to as B0 or the main magnetic field. When a substance, such as human tissue, is subjected to the main magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with the main magnetic field. MRI data acquisition is accomplished by exciting magnetic moments within the primary magnetic field using transmit RF coils. In particular, these RF coils are pulsed to create RF magnetic field pulses in a bore of an MRI scanner to selectively excite a volume corresponding to the region of interest in order to reconstruct MR images of the region of interest using signals received by the RF coils.
There are a variety of techniques used to determine if the B1 field produced by a magnetic resonance coil or array is homogeneous or to what degree the field is inhomogeneous. Such techniques are often referred to as B1 mapping. In general, B1 mapping techniques may either implement spatially or non-spatially resolved B1 measurements. B1 measurements are spatially resolved if one or more spatial encoding gradients are applied during acquisition and, in contrast, B1 measurements are non-spatially resolved when spatial encoding gradients are not utilized during B1 measurements. B1 maps can be used, for example, to adjust transmit gain to produce an RF pulse at a specific flip angle, to design multi-transmit channel RF pulses, to aid in the implementation of chemical shift imaging, to correct images for shading resulting from B1 inhomogeneity, to predict tissue heating distributions, or to calculate electrical properties of tissue, such as permittivity or conductivity. Some B1 mapping techniques are also T1 dependent. That is, the signal utilized for B1 is often weighted as a function of T1 relaxation. Other B1 mapping techniques are B0 or chemical shift dependent. Still other techniques are inaccurate over certain ranges of the B1 field, and/or are dependent on large RF power depositions.
Additionally, phase-based B1 mapping techniques are known and use the phase accrued from a 2α-α flip angle sequence to determine B1. Although such a technique is more accurate than other techniques over a larger range of flip angles, such a technique is B0 dependent and often relies on a relatively long repetition time (TR). Other phase-based B1 mapping techniques utilize a B1-dependent phase produced by adiabatic hyperbolic secant half and full-passage pulses. However, the specific absorption rate (SAR) associated with such techniques can limit the clinical application of such techniques at a high magnetic field.
B1 mapping methods for MRI systems having multiple transmit coils have additional constraints. For example, multiple B1 fields from multiple transmit coils must be measured in a clinically acceptable scanning time, and B1 fields in such a system are typically less homogeneous than in MRI systems having a single transmit coil. B1 mapping methods for a system with multiple transmit coils typically must also have good performance over a wide range of B1 fields.
Thus, conventional techniques for B1 mapping may have limitations over particular field ranges or require properties that limit the clinical applications of such methods.